In the known NMR pulse sequence referred to as the echo planar imaging (EPI) method, nuclear magnetic resonance signals are generated in rapid sequence as gradient echoes by continuously changing the polarity of a readout gradient. Due to the changing polarity of the readout gradient, the samples acquired from the nuclear magnetic resonance signals must be sorted upon entry into a raw data matrix such that the sorting direction changes from row to row of the raw data matrix. If only slight deviations occur from row to row, however, this leads to so-called N/2 ghosts, i.e. the actual image, given an image matrix of N.times.N points, is imaged again, generally with a different intensity, shifted by N/2 in the positive or negative direction with reference to the middle of the image matrix.
Several methods are known for solving this problem. U.S. Pat. No. 5,138,259 discloses a method wherein an adjustment scan is implemented before the actual measurement. This adjustment scan differs from the measurement scan only in that it is implemented without phase coding. A complete correction data matrix is acquired and from which various inadequacies of the measuring system can be determined and correction data for the actual measurement can be calculated. The overall measuring time, however, is lengthened with this method.
U.S. Pat. Nos. 4,644,279 and 4,970,457 disclose acquisition of a nuclear magnetic resonance signal, with (i.e., unactivated), at least one of the magnetic field gradients suppressed before the actual measurement, and this nuclear magnetic resonance signal is utilized for determining the basic magnetic field. Image disturbances that are produced by changes in the basic magnetic field can thus be eliminated.
Proceedings of the Society of Magnetic Resonance in Medicine, Vol. 3, 12th Annual Scientific Meeting, Aug. 14-20, 1993, page 1239 discloses a method wherein a phase-coding pulse is omitted in the middle of the k-space. Rows acquired in different directions of the k-space, that are employed as reference values, are thereby obtained. After a Fourier transformation, relative time shifts are determined from the reference rows in a two-stage method. The image data are corrected on the basis of the correction data acquired in this way. This method for acquiring the correction data, however, is rather complex and the omission of a phase-coding step leads to a discontinuity in the data acquisition that can negatively influence the image.
IEEE Transactions on Medical Imaging, Vol. MI-6, No. 1, March 1987, pp. 32-36 discloses a phase correction method but does not refer to the initially explained problem in the EPI method. Phase errors of the first order are thereby estimated by autocorrelation of the complex phase distortions of the image data, whereas a phase correction of the 0.sup.th order is acquired from the histogram of the phase distribution of the image corrected with respect to the first order.